Devices that detect the presence or absence, the position, or the like of an object with the use of milliwaves and microwaves as detection media have been widely used in various application fields. For example, Doppler radars that use microwaves or milliwaves are used for safe driving or automatic driving of a vehicle. In addition, Doppler sensors utilizing radio waves are used as intruder detection sensors that detect an intruder in a house or the motion of a person in a room. In the production sites or work sites in factories, in order to detect the presence or position of manufactured goods, measurement devices utilizing microwaves are used.
Patent Document 1 discloses an example of such a technology as uses radio waves to measure the presence of an object or the position of an object, in which a transmitter that transmits radio waves and a receiver that receives the radio waves transmitted by the transmitter are installed with a constant distance therebetween, and a transmission sensor that judges whether there is an object between the transmitter and the receiver or not with the use of the radio waves propagating through a space between the transmitter and the receiver is also installed. In addition, Patent Document 2 discloses a technology in which a transmitting device transmits a high-frequency signal in 24.2 GHz band to a predetermined detected area as a pulsing microwave, and a receiving unit detects the signal and detects the presence of a mobile object between the transmitting unit and the receiving unit.